Weymouth originated as a settlement on a constricted site to the south and west of Weymouth Harbour, an outlying part of Wyke Regis. The town developed from the mid 12th century onwards, but was not noted until the 13th century. By 1252 it was established as a seaport and became a chartered borough. Melcombe Regis developed separately on the peninsula to the north of the harbour; it was mentioned as a licensed wool port in 1310. French raiders found the port so accessible that in 1433 the staple was transferred to Poole.

Melcombe Regis is thought to be the first port at which the Black Death came into England in June 1348, possibly either aboard a spice ship or an army ship. In their early history Weymouth and Melcombe Regis were rivals for trade and industry, but the towns were united in an Act of Parliament in 1571 to form a double borough. Both towns have become known as Weymouth, despite Melcombe Regis being the main centre. The villages of Upwey, Broadwey, Preston, Wyke Regis, Chickerell, Southill, Radipole, East Radipole and Littlemoor have become part of the built-up area and are redirected here.